MIPS32R2: Fix MethodLoadKind::kBootImageLinkTimePcRelative

This makes MIPS32 boot again.

The issue was introduced in commit
6597946d:
Static invokes in slow paths would sometimes get
HMipsComputeBaseMethodAddress from the stack into the
same register where the art method pointer would later
be loaded (A0) with the former being overwritten in the
process of loading the latter.

Test: booted MIPS32R2 in QEMU

Change-Id: Ib584cf66795574175650f42b191c797fb3b3965f